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Old 10-30-2007, 03:30 PM   #4
Scooby Specialist
Member#: 56468
Join Date: Mar 2004
Chapter/Region: NWIC
Location: Seattle, WA
04 Improved STI
Dirty White


Lots of good information there.

I'd like to add a couple of things, because tuning boost is integrated with everything else.

Before I do any WOT boost tuning I take about 4 degrees out (numbers are lower) so you are not detonating. There's not much point in tuning the boost unless you are NOT detonating. Don't worry about EGT while tuning too much it will be higher, but you won't burn a valve with a few pulls.

In general I start by removing about 5% of the fuel from the top load sites (I also rescale the fuel map for the increased boost). In general the stock fuel map is very rich. Next I remove timing, say 4 degrees, or about -20%. What you do with the dynamic advance table is a matter philosphy (where the multiplier comes into play), suffice to say the stock route is not recommended.

Now you are ready to start tuning fuel. You need a wide band sensor that is calibrated. It should read 14.7 while cruising (you should have already sorted out your injector calibration and MAF calibration).

You can tune fuel to the stock boost settings and then work your way up boost. This is the conservative approach. You are mainly "tuning" the wastgate duty to increase boost. You can alter the turbo dynamics, but really, what's the point unless you reach target boost. The dynamics are really about how the solenoid responds or how it arrives at the target. For sure, the more "proportional" or "gain" you have, both positive and negative, the more aggressively the solenoid is pulsed to reach the target. The AFR that you tune to is a judgement call. The lower the AFR the less power you'll get up to about 12.5:1. If you are new to tuning, I'd shoot for 10-10.5:1 AFR.

So, let's say you get the fueling dialed in with a constant stock level of boost, say 13 psi, all the way to redline. Now you can raise the boost and tune the fuel again. Go up in 2 psi increments. On the stock turbo, you will hit the limit of the compressor. Boost will start to taper with the stock solenoid no matter what WG value you set. With a aftermarket solenoid you can bleed off more air and hold boost pressure longer, but at the expense of compressor efficiency. IMHO, don't bother trying to hold on to boost with a small turbo. Let it taper. You are not going to get any more power by holding the boost pressure, because the air is just a lot hotter. You can prove this by looking at your MAF readings--no more air is going in.

So how high do you tune the boost? When do you know when to stop? It's a very subjective but there are some clues.

1) If the boost is tapering despite increasing WG duty at high RPM, this is where I'd stop. You can get more peak boost, but then your car drives wierd because the mid range has so much more torque. So, when it tapers a couple of psi from 5500 RPM to redline. That's your max boost.

2) What if you have a big honker turbo and you can hold 30 psi of boost all the way to redline. How do you decide then? You can't run 30 psi on pump fuel. Then its a combination of boost, detonation, and timing (fueling can come into play as well, but I don't use fuel to suppress detonation above a certain AFR). The max boost you can run with a larger turbo is the boost pressure where the engine starts to detonate and the timing still allows EGTs of less than ~1700F. Then I'd lower the boost by 1 psi and take a degree or two off of the timing.

People who tune all the time develop a pretty good guess of where the car will end up. They can tune faster because they can take bigger steps and fewer ones. If you are just starting to tune your own car, go slowly, take small steps. Don't change values by more that a few percent----and keep an eye knock at all times.

Freon has a great thread on tuning the turbo dynamics for an aftermarket solenoid. I can't remember if it's here or there (Enginuity). He did a lot of trial and error work to really figure out how changing values affects the boost response.

In general tuning goes fuel-->boost-->timing in that order. Except with large turbos that are capable of much more air flow than pump gas can handle, then there is often a boost-timing cycle that iterates until you reach the maximum boost with low enough EGTs.

Better to start with rich fueling, LOW AFR, and work your way up. Better to start with retarded timing, LOW values, and work your way up. Better to start with LOW boost and work your way up. See the trend careful!!
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